Telecommunication exchange system



J1me 1953 G. c; HARTLEY ETAL TELECOMMUNICATION EXCHANGE SYSTEM 14 Sheets-Sheet 1 Filed April 29, 1949 INVENTORS GEORGE c. HARTLEY DONALD A- WEM ATTORNEY June 1953 G. c. HARTLEY ETAL 2,540,372

TELECOMMUNICATION EXCHANGE SYSTEM Filed April 29, 1949 '14 Sheets-Sheet 2 SUBSZAT/ON LIN LINE CIRCUIT u/w FINDER sk/ 5L- NMRB I a k2 Q 0-|: 5 ;A m, MAG MR4 1'2 2 I a E 2 2721 i L :3: c Q P sk3 SELECTOR b SUPY d 0 M2. k

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Filed April 29. 1949 I4 Shee ts-Sheet 3 FIG. 3 A.

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TELECOWMJNICATIQN EXCHANGE SYSTEM Filed April 29. 1949 14 Sheets-Sheet 5 F/G.4A k

h k 2 Q h 5 4 :tii 3* m I a $2 a w: 2 a'% a I 2w m 3% MN Wk 4 TIME BASE START INVENTORS 150 965 a. HARTLEY DOA/4L0 A, WEIR ATTORNEY June 1953 G. c. HARTLEY ETAL 2,640,872

TELECOMMUNICATION EXCHANGE SYSTEM Filed April 29. 1949 14 Sheets-Sheet 6 CALLING PARTY OR STORE UNIT z 'm- 3 i i q [u OCC/ hsul M su/ wcl occ 4 '3 E] s I 5 P0455 mscsoss (r4 MESSA 0 Q as occ arc mg Z PUL$E FOLLOWS Mess/16E DETECTOR mr:

T0 FIG 4A is sue)r LR 4 fi/v TYPICAL CASE u? OPERATES WITH FIR-S7 DIG/T B) INVENTORS GEO/F65 C, HARTfY DONALD A. WEIR BY ATTORNEY June 1953 s. c. HARTLEY ET AL TELECOWUNICATION EXCHANGE SYSTEM Filed April 29. 1949 14 Sheets-Sheet 9 GEORGE C, HARTLEY DONALD A WEIR BYg.

ATTORNEY June 1953 G. c.. HARTLEY ETAL 2,640,872

I TELECOMMUNICATION EXCHANGE SYSTEM Filed April 29, 1949 l4 Sheets-Sheet 12 TI'TmuUTlIIU-IT INVENTORS GEORGE C. HARTLEY DONALD '4- WEIR vBYZ ATTORNEY J1me 1953 a. c. HARTLEY ETAL TELECOWUNICATIQN EXCHANGE SYSTEM Filed April 29, 1949 14 Sheets-Sheet 13 INVENTORS m E w A D m m 0 ATTORNEY June 1953 a. c. HARTLEY ETAL 2,640,872

TELECOMMUNICATION EXCHANGE SYSTEM Filed April 29. 1949 14 Sheets-Sheet 14 Grail '1 SEL ATTORNEY Patented June 2, 1953 UNITED STATES ATENT OFFICE TELECOMMUNICATION EXCHANGE SYSTEM Application April 29, 1949, Serial No. 90,322 In Great Britain April 30, 1948 13 Claims.

This invention relates to telecommunication in exchange systems and more particularly, though not exclusively, to teleprinter exchange systems.

The main object of the invention is to ensure the setting up of a correct connection before communication takes place, and to ensure that appropriate action is taken if an incorrect or an alternative connection is made or no connection at all can be established.

According to a main feature, the invention comprises a telecommunication exchange system in which a station identity signal is sent from one point to another a, plurality of times in different, but related, code forms, means for receiving the said identity signal transmissions and for recording them in a single code form, whereby the subsequently received identity signals may be compared on receipt with the first recorded identity signal, and means for exercising difierent control of subsequent operations according as the said signals agree, or do not agree, in identifying the said station.

According to another feature, the invention comprises telecommunication exchange equipment arranged to receive a plurality of station identity signals transmitted in diiierent, but related, code forms over the same channel, to record said received signals in a single code form and to compare them, and thereafter to exercise different control of subsequent operation accordingly as said signals identify, or do not identify, the same station.

In this invention, a code form is intended to mean the expression of a character (a letter or figure, for example) in a particular coding system, and different code forms as applied to one character means different expressions of that character in the same coding system. P,elated code forms means that the diiierent code forms for a particular character have been derived "one from the other, e. g. if the representation of a character in a -element constant total permutation code system of marks (positives) and spaces (negatives) (a binary system of coding) is SMMSM then a related code form is MSSMS (++-)-the binary complement forinand another is MSMMS reversed code form.

It is unlikely that a signal transmitted in one code form and subjected to reversals on the line or in equipment to one or more of its component elements will be affected exactly similarly in its derived (related) code form(s), and hence a signal transmitted in two or more related code forms which are reduced to a standard form on receipt should agree between such reduced code forms. If there is disagreement, this may be taken as evidence that one or all of the code forms has been subjected to maltreatment during transmission and that none can be relied on for correctness, and hence that checking is required.

The double check as described is only provided for other than local calls not involving a junction, and is used to ensure correct transmission over a junction (or a series of junctions) from an originating switching center to a terminating switching center. For local connections, including the connections to the local ends of junction calls, simple WRU? checking is reliedcn.

With this introduction to the object of the invention it is permissible to pass to a description of a specific embodiment of the invention illustrating the use of related code forms in checking an identity signal, and taking appropriate action as a result of such a check.

The invention will be particularly described with reference to the accompanying drawing illustrating as an embodiment, a, teleprinter switching system employing machine switching with constant total permutation code selection and incorporating overflow junction storage. The arrangement to be described has been purposely kept simple to enable a ready understanding of the principles involved to be made, but the invention is not, of course, limited to the arrangement shown.

In the drawing- Fig. 1 illustrates in block form a trunking diagram with junction storage of the system 'to be described;

Fig. 2 shows the essential details of a calling subscribers station equipment and exchange'line circuit, with connections to a line finder bank;

Figs. 3A, 3B and 30 show further details of the exchange equipment, with a group selector, a final selector and part of the register associating circuit;

Figs. 4A and 4B show part of a register;

Figs. 5A and 5B show the receiving and storing section of the register, while Figs. 6A and 6B show the same portion of the register as Figs. 5A and 5B but bringing out the reading and transmitting functions while suppressing the actual storage functions. Figures 5A, 5B and 6A and 6B are complementary to one another;

Figs. 7A, 7B and show the register functions in skeleton form, Fig. 7A the receiving and storing function, Fig. 7B the reading function, and Fig. 7C the transmitting function;

Figs. 8A and 8B are included to illustrate typical outlet allocations for a final selector to enable a clearer appreciation to be had of the detector circuit of Fig. 9;

Fig. 9 shows the detailed circuit of the detector illustrated as a block in Fig. 4B; and

Fig. 10 illustrates the storage unit control circuit.

In the drawings, the relays are shown in accordance with the detached contact method, the contacts being shown close to the circuits which they control and not necessarily next to the corresponding relay coils. The relay coils are marked with capital letters and the corresponding contacts with the same small letters and in addition, a numeral distinguishing the various contacts of the same relay. The numeral appearing under a capital letter associated with a relay coil indicates the number of contacts controlled by the relay.

Before embarking on a detailed description of the embodiment, it will be desirable to consider some general aspects of the problems met with in number checking, and for this purpose reference should be had to Fig. l.

The functions concerned in number checking are conditioned by the type of application concerned and may be listed as under:

(a) Calling party directly connected to local called party Referring now to Fig. 1 the calling party, indicated as a teleprinter TP, will gain access to a regular connecting circuit via the line finder and line circuit shown, and via this to a register in a manner well known in the telephone switching art and will then discharge into the register the selection information. Here the permutation code is translated into a series of markings and the switch train set up accordingly, via the group selector and final selector shown, to the bank multiple of which the local teleprinters are connected. Connection to the called party promotes the transmission thereto of Figure ShiftD (the Who are you? or WRU? request of the International Alphabet No. 2

(C. -C. I. T.)), which causes the called partys answer-back unit to function and send his directory code number to both the register and the calling party over the path as just set up.

In the register the returned code is checked against that originated by the calling party and stored in the register. If the two codes are identical the register releases and allows the connection to proceed; but if the two codes do not agree, a message is returned to the calling party to advise that a wrong connection has been made.

Should the local number required prove busy the detector operates a relay OCC (q. v.) and this returns the signal DOC to the calling party to advise him of the condition.

(1)) Calling party directly connected to called party via a junction For this case Fig. 1 is to be read in its aspects both as an originating end and as a termi nating end. At an originating end, a calling party (TP) is connected by means of the register (originating register) and group selector to an outgoing junction either direct, via the junction routing switch (Juno. R. S.) or via a storage unit, as shown. At a terminating end, the outgoing call (above) on the junction is received over an incoming junction and connected via an incoming connecting circuit and associating switch to the register (incoming or terminating register). From these the call is set up in much the same way as for (a) with, however, some slight differences.

The conditions in this case are somewhat governed by the inability of the incoming register at the terminating end to determine whether the call originates from storage or is a direct connection.

When the junction is claimed by the originating register the receive line of the junction remains at space until the terminating register is picked up when the condition goes to mark. The originating register then transfers the code and its binary complement (i. e. mark for space, and space for mark) to the terminating register and restores. Y

The terminating register checks the code received. If the code is incorrect, the register returns the signal WC to the calling party and also connects the circuit to an operators position (shown as Manual Board). This latter is to cater particularly for a call incoming from storage and it is therefore arranged that the call signal at the operator's position is delayed by a predetermined time to give the calling party in a live connection the choice of clearing without calling in the operator unnecessarily. The operator would be necessary, of course, in the case of a message from storage and the delayed WC" signal at the manual board would in thatcase eventually mature.

If the code is correct, selection proceeds from this point as for (a) and the called partys line is reached. If the called partys line is engaged the signal OCC is returned to the calling party and the register effects, as before, delayed connection to an operator to cater for the case of receipt from storage.

If the called partys line is free it is seized and WRU? transmitted to trigger the called partys answer-back unit. The returned signals are checked against the signals already held in the terminating register and if they are incorrect, the signal WC is returned to the calling party 1 and delayed connection to the manual board initiated.

If the reply checks correctly the terminating register switches the connection through and releases. It should be noted that the double check transmission of code and its binary complementis only effective over the junction route, between originating and terminating registers.

(c) No junction free at an originating end If no free junction is found by an originating register in (b) the hunting selector continues the search for the particular marking designating that junction but now testing for a free storage unit. If no storage unit is found free the register detects this on the last position of the group and transmits the signal OCC followed by release of the register,

If a free storage unit is found, a mark signal is returned by it to the register to cause transfer of the selection code and its binary complement and the register then functions if it were connected to a real junction.

(d) Connection incoming from junction terminating end It is arranged that no discrimination between a message being received direct or from a storage unit is necessary by arranging that the functions appropriate to the two classes of call are each carried out. For instance if the double check of the code is incorrect the calling party should be advised in the case of an original (live) call, while in the case of a call received from a storage unit the message must receive manual attention. It is arranged that the storage unit control circuit will be unaffected by the messages returned for a possible calling party and that delay will be introduced in switching to the manual board to allow the calling party, if present upon the connection, to clear before the operator is introduced.

Thus the functions of the circuits under incoming conditions differ from local only in that the selection code must be checked upon receipt. A separate start circuit is used to indicate the corn dition and thus arrange for conversion of the binary complement of the code to straight code.

The circuits will now be described in detail with reference to the relevant figures.

(a) Calling party directly connected to local called party The teleprinter station equipped with a calling key, a clear key, a supervisory lamp and facilities for the control of the teleprinter motor, as shown in Figure 2. When a party wishes to make a call, key KC is operated which reverses the line potential and so operates line relay SL at the exchange, and this, in turn, causes the association of a register with a connecting circuit and a line finder. The line finder magnet LFM is driven from 81! in well known manner until the calling line is reached when it is halted and relay SK of the line circuit operated via $22 from the earth extended over the E are of line finder LF. SK operating disconnects the SL relay at ski and switches the lines through at ski and SM to the connecting circuit and register, Figures 3 and 4 respectively. The calling line is busied on the final selector multiple at SM and ski.

Machine start relay MS (Fig. 2) in the calling station is now operated to earth by positive battery returned by the register over the R lead by the following path:

Earth, teleprinter magnet MAG, relay MS, rectifier MRA, sIcZ up, are R of line finder LF (Fig. 3A), swb2, lead RC (Fig. 43), lead RC, 0003, su t, 2005, hsut, esa2, lr i, TRC in M position, to positive potential.

Relay MS operates at msl the slow release relay MSR, which in turn starts the motor of the teleprinter via msrl and msr2 and lights the supervisory lamp at msrt. MSR. also by-passes the make-contacts of calling key KC, at ms'rt, so as to maintain battery on the send line when the key is released.

The calling party may now transmit the selection information by permutation code to the reg ister associated with the circuit. The TX contacts (Fig. 2) which are associated with the subscribers teleprinter, transmit the intelligence in double, current signals, positive potential repre senting a mark and negative potential a space, over a circuit which includes contacts $70! up, are S of line finder LF, lead S, swbl and lead SC of Fig. 3A, lead SC of Fig. 4A, esl, ccal, polarised relays TRB and TRA to earth, the latter via cca2.

is assumed to be These signals are responded to by relays IRA and TRB which respectively repeat the signals into the receiver portion of the register (Figs. 5 and 7A) and start the time base, indicated by a block in Fig. 7A. The time base is not shown explicitly but may be assumed to be similar to that described in the copending application of D. S. Ridler-L. R. Brown, bearing Serial No. 85,789 and filed April 6, 1949, now U. S. Patent 2,570,279.

The function of the time base is to generate two series of equally spaced pulses in response to the start impulse, the pulses of one seriesthe counting pulsesbeing interleaved with those of the other series-the scanning pulses. The time base runs until it is stopped by a stop pulse specially applied. The functions of the two trains of pulses are fully described in the application hereinabove referred to, and will appear from what follows.

The reception of the selection code will now be described.

When the register and regular connecting cir cuit were associated, relay ST in the register (Fig. 4A portion) operated to prepare the circuit for the receipt of the code over a circuit which may be traced from ST via the Normal Start wire strapped, in Fig. 3A, to swam, cll to earth. ST holds on its right-hand winding to earth via stl up, el normal.

ST has been shown throughout the various figures as a relay having 1 5 contacts. In prac" tice, 8 of these would be taken over by a relief relay to ST.

The constant total teleprinter signals used in this system are the usual 5-element signals of marks or spaces (the intelligence elements), together with an initial start element, which is always a space, and a final stop element, which is always a mark.

The starting space element operates TRA. and TRB to space, the operation of the latter starting the time base over a circuit which may be traced in Fig. 4A from earth over $1.03 normal, bczl normal, 'IRB at S, arl normal (see also Fig. 7A), the operation of the former, in Fig. 5A, being ineffective as the scanning pulses have not yet been generated.

Figure 7A shows diagrammatically the arrangement of the receiver portion of the register and Fig. 5A shows this portion in greater detail. Tubes UDA to UDG are connected as a scale of seven counter and form the unit counter or distributor operating in the manner described in the aforesaid copending application Serial No. 85,789, Patent 2,570,279 dated October 9, 1951. Its function is to advance from UDA forward, one tube each time a counting pulse is received from the start-stop time base, each tube as it fires preparing the next tube for firing and extinguishing the previous tube. ST in operating, triggers UDA (Fig. 5A) by change-over of the make-before-break contacts stll generating a positive impulse so that the first tube of the dis tributor is struck while the circuit awaits the arrival of a counting pulse, and, in this condition UDA by reason of the voltage drop across the cathode resistor extends a positive bias to prepare the trigger electrode of UDB so that this will be the tube to strike when the first counting pulse is received.

In a similar manner, ST triggers, (Fig. 513), at silt, CDA of the three tubes CDA, CD13 and CDC forming the receive digit distributor and also, at stl2, 002 of the binary pair CCA, CCZ which count the number of cycles of the digit distribu- "tor. 'iFiringipf CCZ-at .thispoint is purelyprepar- :atory. KCDA extends bias from its cathode to prepare-the triggerelectrode of'CDB so thatCDB will operate .in conjunction with UDG upon its next operation and the arrival of a counting pulsaand also extends-positive bias to the control electrodes of the binary pairs SXA, MXA etc. (Figs. 5A and 5B) in the X series of tubes. Details for only one pair of tubes in each of the X, Y and Z series have been shown but the arrangements for the other pairs are exactly similar.

Twenty milliseconds after the starting of the time base by TRB the :first counting pulse matures and is applied over the counting pulse lead (Fig. 5A) to the control electrode of UDB which strikes, -biassing UDC so that it operates on the second counting pulse. UDA is extinguished by virtue of the potential drop in the common anode resistor R1. When UDB strikes, a positive bias is extended from its cathode via MR4,-coa4, and MR3, ca3, respectively, to the rectifier resistance gates formed by MRI, resistances RI, R2 andMRZ, resistancesR3, R4. The scanning pulses occur midway between the counting pulses so that at 30 milliseconds from the start of the time base, a scanning pulse is connected over the Scanning Pulse lead and via coaZ to the junction either of resistancesRlR2 .or of resistances R3-'R4 dependent upon the position of the tongue of relayTRA. If TRA is at mark, then a positive scanning pulse is fed to the trigger electrode of tube MXA and as .this is already in receipt of a positive bias from CDA, the tube fires, recording that the first intelligence element of the first digit was a mark. Other mark tubes of the same digit row, MXB, MXC etc., cannot strike because the positive scanning pulse potential is dissipated through their own RiMRl gates (shown ,multipled at TRAM) as tubes'UDC, UDD etc., are all normal and the relevant'M leads thus at earth potential. Tubes MYA and MZA, although in receipt of the scanning pulses, cannot strike because they are not in receipt-of positive trigger bias (i. e. from CDBandCDS respectively) Had the first intelligence element been a space, the tongue of relay TRA would have been at space, thus-connecting the scanning pulse to the tubes SXA, SYA, SZA, of which SXA would be the only tube to strike.

The succeeding counting pulse triggers UDC so that the tubes SXBMXB are prepared for the receipt of the next scanning pulse.

The digitdistributor proceeds in this manner to record the intelligence elements of the first digit, which is completed with the tube UDF struck. The next counting pulse-fires UDG which .thus extends positive over MR5 to controlthegate formed by resistances RfiwRfi. The previous counting pulses were dissipated in resistance R6 with UDG holding point P at or near earth potential but with UDG now struck, a counting pulse matures at point P and thus triggers tube CD-B via 0003 in the receive digit distributor, CDB receiving bias from CDA. CDB extinguishes CDA via the common anode load resistance R8 and at its cathode, extends bias to CDC and also to the trigger electrodes of tubes SYA, MYA, SYB, MYB etc. in the Y series of tubes, to prepare thenrto receive the elements of the second digit.

The firing of 'UDG also causes a positive potential, stop pulse to be'sent out from its cathode to stop. the time base, and pulsepotential isalso sent 8 back over resistance R9 ",to prepare UDA for operation on the :next counting pulse to arrive. 'I'RB reverts to mark (normal line condition).

The secOnddigit now commences and re-starts the time base. After 20 milliseconds, the first counting pulsematures and operates UDA, after 4:0 milliseconds UDB and so onas before, with IRA this time connecting the scanning pulses to operate SAMY A, SYBMYB etc. As before, when the digithas been received, the next succeedingcounting.pulse strikes UDG and steps the receive digit distributor so that CDC strikes and the tubes ,SZA, MZA, SZB, MZB etc. in the Z seriesof tubes are vbiassed in anticipation of the receiptof the third character. UDG .also prepares UDAoncemoreand stops the time base.

Thethirddigit nowcommences. TRB restarts the time base and the digit is received as described above, and recorded on tubes S ZA, MZA etc. in the Z series of tubes.

Tube CDC also biasses the trigger electrodes of tubes CDA, CCA andCCZinaddition to those vof the Zseriesof tubes, so that when the counting pulse following receipt of the digit is allowed .by UDG to mature at point 1?,- CDAstrikes again and CCA also strikes, extinguishingCCZ. .CCA operates relay CC which is serially connected in its discharge path and which relay is used to indicate that the digits have been received and to prepare the register-for its reading and transmitting functions.

Referring now to Figure 4A, the operation of CC in Fig. 5B completes a-circuit'for relay RD from-battery via-,ic3, ccl up, esa I st3-up, to earth. RD operates, locking over its own contact m2, and operates RDR-over-an obvious circuit via rdl.

Relays RD and RDR connect'the trigger electrodes ofthe reading tubes SRA, MRAetc., in the R series of tubes (Figs. 6A and 7B) to the cathodes of tubes SXA, MXA etc., via leads SA, MA etc., over contacts raid, 1'd5, etc., and the rectifiers like MR6, MR1 and so on. When relay .ST operated upon seizure of the register, it-fired from theXchain of tubes, so that if, for instance,

the SXA, SXB, MXC, MXD and SXE. tubes are struck, there will exist positive potentials on the leads SA, SB, MC, MD and SE due to the X series of tubes alone and these will trigger the corresponding reading tubes 'SRA, SRB, MRC, MRD,,SRE. Consequently relays MA, ME, ME will operate and at their contacts connect an appropriate phase marking to the detector (Figs. 4B and 9). The corresponding potentials in the Yand Z series of tubesare ineffective because TCDB and TCDC are normal and the rectifiers .like MRlO, MRI l, gate these potentials to earth.

It is necessary here to digress a little to discuss the use of phase marking for the control ofautomatic selector-s.

This principle of phase marking for this pur- DQSBIlS disclosed-in the U. S. Patent No. 2,424,585. Briefly a source'of alternating current is transformed into '12 sources having equal phase displacement between eachphase, the whole occupymg 360 degrees. Thus it can be arranged that phase 1 is characteristic of the outlets corre :sponding to digit 1, phase 2 characteristic of the 9 the received digit may thus be connected to a detector and the latter so arranged that only when the phase potential encountered upon the tested switch outlet corresponds with that connected to the detector, will the detector function.

In practice, it is necessary also to determine if the detected contact is free or busy and thus the detector must include facilities for checking the line condition. In addition it must be possible for a selector to be driven over a group of outlets, such as the outlets from a group selector corresponding to the digit concerned or the outlets of lines connected to the same subscribers station. In the circuits shown in the accompanying drawing the selectors used are homing-type single-motion unidirectional switches. In the case of group selectors, the outlets to a particular final selector or junction group are for convenience, arranged to occupy consecutive outlets of the arcs. The contact of the are after the last outlet of a group is not connected to any trunk but bears a positive potential which indicates to the detector circuit that no free trunk has been found in the group concerned. A permanent marking of all the phases is connected to the b outlets on these positively marked contacts so that the detector will always function independently of the marking sought.

In the case of final selection, it is necessary first to select the group of lines designated by the tens digit and then to find the particular line characterised by the units digit. Here it is found convenient to group all lines designated by the particular tens digit on consecutive outlets and also to indicate that all lines in a particular tens group have been tested by providing a spare outlet immediately following the tens group which bears a positive potential in a similar manner to that of group selection. Such segregation of the tens groups is convenient because it allows the searching selector to be arrested after all units associated with the tens group have been searched and thus prevents the necessity to delineate between units associated with the required tens group and units associated with other tens groups on the same are.

To simplify the final selection testing arrangements, the position of lines in the group of tens concerned is conditioned by the tens digit. Figure 8A shows the arrangement where it will be seen that the first line of the particular tens group is that line which has the same units designation. Thus 11 is the first line of the tens one group, '22 that of the tens two group and so on. Figure 8B shows a typical outlet arrangement for a particular tens group. Here 43, 45, 48 and 40 all have more than one line per number so that search over all the lines associated with that number should be made before a busy condition is recorded.

DETECTOR CIRCUIT The principal component of the detector circuit shown in Fig. 9 is concerned with the detection of the phase marking. This is formed by transformers TP and TQ, gas discharge tubes SVI and SVZ, and high speed relay WP, with their associated resistors and condensers.

The phase marking (41! 17 determined by the selection code is connected by the marking relays (MA to ME) (Figs. 6A and 63) to the primary winding of TP (via 1006, ST!) while the primary winding of TQ is connected to the selector under control. While the phases on the two transformer primaries are different a potential will be maintained by transformer action upon the two co-operating cathode electrodes of tube SVi so that this tube will strike from these two.

any of the gaps to cause breakdown. Ho'wever; when the primary of the transformer TQ]encounters the same phase potential as that "con-'1 nected to TP, tube SVI ceases to break down be-' cause the transformer secondary potentials are not sufiiciently diiferent. Thus after the delay caused by the capacity-resistance network in the anode circuit, electrode t of the tube SVZ rises" to"-" ward earth potential, breaks down the control gap and thus the cathode-anode gap so that relay WP operates in a manner to be described 'laterl'f It is now desirable to revert to a discussion of the main processes of group and final selection,

and to return to the details of the operation of the detector circuit later.

Referring now to Figures 3A and 4B, the de'-' tector block extends earth over the a lead and the SA lead to operate relay GA (Fig. 3A) of the group selector via swb3 and ob! contacts normal:- Operation of GA completes the latch magnet (GLM) circuit to drive the switch by earth from the detector (Fig. 4B) over lead 0, swal normal, glad normal and ga2 operated, latch magnet to battery. The detector tests the circuit via lead (Fig. 4A) b, cs3 normal, RA lead, swb'd normal, gb2 normal and are e of the group selector. When a free outlet is detected and confirmed, the detector removes the GA operating earth from lead a so that GB (Fig. 3A) operates in series with GA over out up, 013 up, and holds it, CL having been operated upon the operation of ST (Fig. 4A) via CL lead, st2 up, wc8 to earth. GB claims the final selector and switches the selecting lead forward via gbl and 9712, to the wipers of arcs a and b.

The first digit of the called subscribers code determines the class of call (local or junction) and hence, the first operation of the MA ME marking relays (Figs. 6A, 6B) also completed a circuit for the LR relay (Fig. 4B) as the first digit designated the call as local and this relay conditions the circuit accordingly. LR. operates from battery to earth over a selection of MA ME contacts (as, e. g. in Fig. 9), std up, at, and holds over 12 up, still up.

The detector also operates relay TA (Figs. 41B and 9) which via tal extends earth received over st5 (Fig. 4A) via MRIZ, we l, if, dl, bl contacts normal to operate register relay A to battery.

MYA etc.

Relay A prepares the register relay B circuit at a! and steps on the reading distributor byfiring tube TCDB in Figs. 6A and 7B and thus sets the marking relays MA ME to the pattern on the tubes bearing the second digit i. e. tubes SYA, TCDB Fig. 7B is fired by an impulse generated at the make-before-break contacts a4 (Figs. 6A and 7) on their operation, and con-'- veyed over coa6 to the control electrode of TCDB.

The detector again extends earth over the SA lead to operate relay FA (Fig. 3B) in' the final selector (via swbt, gb l (Fig. 3A) up, are a of group selector, fhl) and releases relay TA. The

release of TA allows register relay B (Fig. 4A) to operate in series with its A relay (over al up) while the FA relay in the final selector connects up the latch magnet FLM via in up, fb3 normal In this condition tube 8V2 cannot strike because there exists insufficient potentialac'ross" to" the are of the group selector. The final selector drives in' search of the tens marking and when this is satisfactorily detected relay TA reoperate's and operates relay C in the register (Fig. 4A) via a similar path as for relay A, but switched at b! up to relay C. C prepares at cl a circuit for operating relay D and steps on the digit distributor as before, by firing TCDC over c2 (Figure 6A) so that the marking tubes (SRA, MRA etc.) now read the value of the third digit, changing the marking relays MA ME accordingly, releasing TA relay and thus allowing D relay to operate in series with its C relay (over 01 up) and causing the detector to again connectthe drive so that the final selector is now set to the units marking. When this is satisfadforily detected the earth is removed from the SA lead 80 that relay FB (Fig. 3B) in the final selector operates in series with FA over fal up, are e of group selector, gb' up, to earth, and the lines are switched through (at fbl, ,fb2).

The earth applied over i125 and the final selector e are operates the called partys CK relay which at 07c! and 0702 extends the line to the ealled party, and at ck3 and c704 removes the phase and battery markings from the final selecw tanks.

The register must now check the called partys line for a correct connection by transmission of the Who are you? code, and the detector therefore functions again, operating relay TA (Fig. 4B). This operates relay E (Fig. 4A) via the same circuit as for relays A and C, but switched now at dl up, and E operating steps the transr'r'iitting digit distributor back to tube T CDA (from a pulse generated at e3 (Fig. 6A) relay TA releases and relay F (Fig. 4A) operates over e1 up, in series with E.

one contact of relay E completes a circuit for relays ES and ESA (Fig. 43) as follows: earth, sit up, (22 up, Zr3 up, relays ES and ESA in parallel to battery. ES is slow to operate and allows ESA to operate first. ESA operating disconfleets relay RD (Fig. 4A) at esal, but owing to the resistive short circuit across its coil this reray is slow to release, so that when ES operates after its short delay, relay ESR (Fig. 4B) operates over earth, es! up, 1113 (not yet released), and locks under the control of relay ZP at 2124 the over its own contact esrZ.

ESR connects the upper winding of the SP relay to the S pulse lead via esr3 up, Z16 up so that it will operate upon the arrival of the first S pulse. When this occurs 'SP operates and locks to earth an its iniddle winding via 2113 normal, is} up, connecting the WRU? signal (Figure stir-n43 derived from a common signal generater (desighated by the letters WRU in the fiuaaie or Fig. 43) via spB up, es'rl up, 114 up, est-z up, to the SA lead and thus to the called party via the group and final selectors a arcs. At the end of the 'cycle of transmission the Z pulse operates relay ZP in series with the lower winding or s? over $121, holding relay SP for the duration of the pulse but causing its release, .2123 being now open, when the pulse ceases. ZP also releases relay ESE. at .2114 so that the cycle is "net again started, and 'ZP is, of course, itself released at $1): how normal.

Receipt of WRU? by the called party trips his answer-back mechanism, causing the return of his directory code to the register via the final and group selector b arcs and ckl up, gbZ up, swb' l rioiinaL RA lead (into Fig. 4A), as! up, TRB rela'y to earth; and from es'l up via cccl normal,

12 TBA relay, ccc2 normal al'soto earth so thatTRA and TRB respond to the signals sent back. The signals are also returned to the calling party over lead RC from lead RA via cs3 up (Fig. 4B) wcl normal, 000! normal, hsul normal, sul normal, RC lead, swb2 normal (Fig. 3A).

Thus the register and the calling party receive the called partys answer back in parallel, the cycle of operation of the receiver portion of the register (Fig. 5A) being exactly as described earlier, noting that upon the operation of CC by the firing of CCA at the end of the reception, the cathode circuit of tube CTT (Fig. 5A) is completed over (:05 up, coed. Tube C I'I may be triggered by pulses derived from the step-up transformer TC, over the primary of which is supplied a common earth feed to all the storage tubes SXA, MXA, SYA, MYA, SZA, MZA, etc. in. the X, Y and Z series of tubes.

Now, as the signals are received, they should coincide exactly with what has already been received as selection information and stored on the X, Y and Z series of tubes, and thus no change of the conditions in the transformer primary should occur, but should the two patterns be other than identical, the tubes in at least one pair will change over their status, causing a disturbance in the conditions in the primary of TC and generating an output pulse from the secondary of the transformer which will trigger tube CTT and thus operate relay WC. WC connects the upper winding of the SP relay (Fig. 413) to the "8" pulse lead at we! and also prepares the RC lead circuit at 2005 so that when SP operates the signal WC derived from the common generator (designated by the letters WC in the middle of Fig. 4B) is transmitted to the calling party via up, wc5 up, su l normal, 0003 normal, RC lead, swb2 (Fig. 3A). WC also breaks the connection between RA and RC at we! so that the message is not extended forward to the called party. When the Z pulse matures and Z? relay operates, as previously described, a circuit is closed for relay Z (Fig. 413) to earth, via 2102 up, wcZ (Fig. 4A). Z operates and looks over its own contact .23 up via $154 up and at zl disconnects relay ST which restores and so returns the circuit to rest, Z releasing after ST has released.

If the returned code checks correctly the cycle will be completed without any change of pattern and thus without firing CTT. CC (Fig. 513) will restore when the check is com-plete because tube CCZ will strike when CDC fires and extinguish CCA, and relay Z will then operate over its righthand winding via cs6 up, 004 normal, 7'1'3 normal, wc3 normal, st! up, to earth. As before, Z restores the register to normal but incidentally extends an earth to relays SWA and SWB (Fig. 3A) over lead SW from :42 up (Fig. 4A) to opcrate these switching relays in series to complete the through connection and allow the register to drop out. SWA and SWB lock to earth via came up, 012 up, and CL relay is held over swa5 and contact tr! relay TR at mark. Switching through connects relay TR to the send line and as this is normally at mark, relay CL is maintained.

The arrangement TR-CL is provided for the detection of the clear signal, constituted by a long space condition on the calling line generated when the calling party clears by pressing KCL (Fig. 2). MSR restores and extends negative potential (space) on the S lead via KC normal, mrsc normal, TX at M. Thus TR moves its t'm'! contact to space and breaks the circuit to CL 

